The present invention relates to a magnetic carrier for use in a two-component developer that is applicable to the methods of dielectric photography, electrostatic printing and electrophotography. In particular, the present invention relates to a lightweight magnetic carrier for use in a developer having a longer service life and having excellent developing properties.
The conventionally known methods for developing an electrostatic latent image formed on a photosensitive drum include the magnetic brush method, fur brush method, pressure developing method, cascade method and the like. However, because of the quality of images obtained, the magnetic brush method is most widely practiced.
In the magnetic brush method, resin particles known as toner are first charged triboelectrically, i.e., the charge is generated by means of friction exerted between the particles. The toner is then allowed to be carried by a turf which comprises of a carrier (for example, iron or ferrite particles) and which is formed on the surface of a sleeve having a magnet disposed inside thereof, whereby the toner powder is transported to the surface of a photosensitive member. Subsequently, the toner is electrostatically deposited on an electrostatic latent image having a negative charge and formed on the photosensitive member, whereby the image is developed. Accordingly, the developer used for this purpose comprises two components; toner and carrier.
Iron powder or treated iron powder used as a carrier have unstable electrical properties, which contribute to a deterioration in image quality. This deterioration is due to a hard turf which is attributable to a highly saturated magnetization. Additionally, iron powder has a larger specific gravity, which necessitates a larger energy for triboelectrical charging. The heat generated by an increased rotational torque allows the toner to readily adhere to the surface of individual carrier particles. As an improved iron carrier, a ferrite carrier has been proposed. However, a ferrite carrier requires a complicated manufacturing process. Additionally, though it is lighter than iron powder (2/3 the weight of iron powder), the ferrite carrier does not necessarily satisfy the requirements for a smaller and more energy-efficient copying machine. Furthermore, such a type of carrier has a wider dispersion in magnetic properties due to a wider variety of particle sizes.
To make the carrier lightweight, methods to form a hollow in individual ferrite particles were disclosed in Japanese Patent Laid-Open Publication Nos. 177160/1982 and 23032/1983. According to these methods, the conditions for spraying, drying and the like should be strictly controlled. A minimum deviation in the conditions results in carrier particles having a wider density distribution. When employed in a developing apparatus, carrier particles having less density and toner particles may be thoroughly rubbed together. In contrast, carrier particles having greater density and toner particles are not satisfactorily rubbed together, which allows the toner to have a larger triboelectrical distributon, and which greatly deteriorates the image quality.
To ensure improved image quality, smaller-sized carrier particles are required. To prepare smaller-sized carrier particles, Japanese Patent Laid-Open Publication No. 66134/1979 disclosed small-sized carrier particles, wherein minute magnetic particles are dispersed in a binder resin. With such a magnetic dispersion type carrier, it is difficult to uniformly distribute the magnetic particles. Consequently, the magnetic particles are irregularly distributed on the surface of the carrier, resulting in uneveness in both magnetic properties and electrical properties. After an extended period operation, the binder resin is selectively worn away, and the surface properties, especially the electrical properties of the carrier, vary, thus resulting in deteriorated developing properties.
Japaness Patent Laid-Open Publication No. 34902/1986 disclosed magnetic particles in which hydroxide and/or oxide of iron are deposited on individual porour polymer particles, onto which a polymer film is further formed. This type of magnetic particle is based on the simple deposition of hydroxide and/or oxide of iron, and consequently, the magnetic substance on the particles may be stripped off. For this reason, it is necessary to form a polymer film after the magnetic substance has been deposited.
Furthermore, Japanese Patent Laid-Open Publication No. 93603/1986 disclosed a method wherein individual magnetic particles are provided on the surface thereof with a magnetic powder by using the thermal behavior of core particles. In this method, though the magnetic powder securely deposits on and in the vicinity of the surface of individual polymer particles, there is little, if any, possibility of mutual bonding among the fine magnetic particles. Consequently, the amount of magnetic powder deposited on individual polymer particles is limited, therefore control of magnetic properties (which is a vital requirement of the carrier) is impossible.